1. Field of the Invention
The present invention relates to an adaptive antenna apparatus provided with a controller for controlling to select a best demodulated signal. In particular, the present invention relates to an adaptive antenna apparatus for adaptively controlling to receive radio signals using a plurality of antenna elements, where the adaptive antenna apparatus is provided in an apparatus such as a portable radio communication apparatus of a mobile communication system to keep higher communication quality, and the present invention relates to a radio communication apparatus using the adaptive antenna apparatus.
2. Description of the Related Art
FIG. 32 is a block diagram showing a configuration of an adaptive antenna apparatus according to a prior art, which is disclosed in a specification of U.S. Pat. No. 5,710,995 (referred to as a Patent Document 1 hereinafter) and Japanese patent laid-open publication No. 10-209890 (referred to as a Patent Document 2 hereinafter). This adaptive antenna apparatus is an example of a configuration in which an adaptive antenna and a selection diversity antenna are combined. By switching over between the adaptive antenna and the selection diversity antenna, it is possible to keep respective signal qualities of signals received by the antenna apparatus at higher signal quality.
Referring to FIG. 32, the adaptive antenna apparatus is configured by including two antenna elements 111 and 112, two processing circuits 113 and 114, two detectors 115 and 116, a signal quality monitoring circuit 117, and a selector circuit 118. First of all, respective radio signals received by the antenna elements 111 and 112 are inputted to the processing circuits 113 and 114. The processing circuit 113 executes an adaptive control processing on inputted radio signals, and thereafter, outputs a resultant signal to the detector 115 and the signal quality monitoring circuit 117. In this case, by suppressing an interference wave in a received radio signal, the processing circuit 113 keeps signal quality of the received radio signal at higher signal quality. Namely, the processing circuit 113 exhibits a great effect when delay waves or cochannel interference waves from adjacent base stations arrive. In addition, the processing circuit 114 executes a selection diversity processing on inputted radio signals, and thereafter, outputs a resultant signal to the detector 116 and the signal quality monitoring circuit 117. In this case, the processing circuit 114 selects a radio signal having higher received power from the radio signals received by the antenna elements 111 and 112, respectively, so as to keep the signal quality thereof at higher signal quality. Namely, the processing circuit 114 exhibits a great effect when the received power fluctuates greatly, such as fading.
In this case, the signal quality monitoring circuit 117 judges signal quality of a baseband signal which is obtained by demodulating a radio signal adaptively controlled by the processing circuit 113, and signal quality of a radio signal subjected to the selection diversity processing by the processing circuit 114. The selector circuit 118 selects a baseband signal from the detector 115 or 116, which corresponds to a signal having a higher signal quality, and outputs a selected baseband signal to an output terminal 119. The adaptive antenna apparatus configured as described above can solve both of the interference wave and the fading, that are two major factors of degradation of signal quality of a received signal in a mobile communication system.
The above-described adaptive antenna according to the prior art has the following disadvantageous problems. In this case, using a radio communication apparatus 90 shown in FIG. 18, such as a cellular telephone including two antenna elements 91 and 92, as a comparative example, a simulation made by the inventors of the present invention and results of the simulation are described below.
Referring to FIG. 18, the radio communication apparatus 90 is provided with an antenna element 91, which is a monopole antenna, so that the antenna element 91 is contained in a rectangular parallelepiped radio communication apparatus housing 90A and protrudes from an upper surface of the housing 90A in a vertical direction. On the other hand, the antenna element 92, which is an inverted F-type antenna, is provided to be parallel to a side surface of the radio communication apparatus housing 90A. In the antenna element 92, a radio signal is fed to a feeding point 93a approximately at a central portion of one side of the antenna element 92 through a feeding line 93. A short-circuit line 94 is connected to a short-circuit point 94a approximately at a central portion of one surface of the antenna element 92 from a short-circuit point 94b of the radio communication apparatus housing 90A, so as to be short-circuited. In this comparative example, such a case is considered in which the radio communication apparatus 90 is inclined at 30 degrees to a horizontal direction 99, as shown in FIG. 19.
For example, when a frequency of the radio signal is 2 GHz, a wavelength of the radio signal is 150 millimeters. When a length in a longitudinal direction of the housing 90A of the radio communication apparatus 90 shown in FIG. 18 is 125 millimeters, the length of the housing 90A is close to the wavelength of the radio signal. In this case, a radiation directivity often shows a directional pattern having an angle (referred to as a null hereinafter) with a greatly dropped received power, since the housing 90A is small in size. For example, FIG. 20 shows a directional pattern of the antenna element 91, and FIG. 21 shows a directional pattern of the antenna element 92. In FIG. 21, received power of the antenna element 92 is normalized based on maximum received power of the antenna element 91 shown in FIG. 20. As apparent from FIG. 21, the directional pattern of the antenna element 92 has two nulls. In the comparative example, such a case is considered in which a desired wave arrives from a direction at zero degrees with respect to a -Y axis direction on an XY plane and an interference wave arrives from a direction at 210 degrees on the XY plane.
Such a case will be considered below, where a multiplier 95a multiplies a radio signal received by the antenna element 91 by a weighting coefficient W1, a multiplier 95b multiplies a radio signal received by the antenna element 92 by a weighting coefficient W2, and thereafter, a signal combiner 96 combines resultant radio signals to obtain a radio signal by an adaptive control circuit shown in FIG. 22. In this case, for example, FIGS. 23 to 29 are obtained as results of a simulation, in which in a directional pattern of the antenna element 91 shown in FIG. 23 and a directional pattern of the antenna element 92 shown in FIG. 24, the above-mentioned desired wave and interference wave arrive, and adaptive control is carried out so that a null is formed in a direction of the interference wave for the antenna element 92. In FIGS. 23 to 28, received power of each of the radiation patterns is normalized by using each maximum received power. As is apparent from these results shown in FIGS. 23 to 29, a bit error rate during the adaptive control is larger than a bit error rate of the single antenna element 92. Namely, the simulation results are summarized as follows.
For example, such a case is considered below in which one desired wave and one interference wave arrive and these two waves are equal in magnitude. Further, when the interference wave arrives from an angle with null of the directional pattern of the antenna element 92, the antenna element 92 does not receive any interference wave and receives only the desired wave. On the other hand, the antenna element 91 receives both of the desired wave and the interference wave. The adaptive array antenna apparatus controls signal components of the interference wave received by the two antenna elements 91 and 92 to be equal in magnitude and opposite in phase, and cancels interference wave components. This leads to higher signal quality being obtained. Namely, when only one antenna element receives the interference wave, an interference wave suppression effect due to the adaptive control cannot be obtained. On the other hand, since the antenna element 92 receives only the desired wave without receiving the interference wave, higher signal quality can be obtained. Namely, by performing the adaptive control, the signal quality often becomes lower than that when a single antenna element is used.
On the other hand, the above-described adaptive antenna apparatus according to the prior art executes the adaptive control and the diversity control. However, higher signal quality cannot be obtained in such a situation, Namely, under the diversity control, the first antenna element that receives both of the desired wave and the interference wave is often selected so as to select a signal larger in power from received signals (paragraph 0019 of the specification of the Patent Document 2 discloses that a diversity judgment criterion is either an average power or an average signal power). In such a situation, the signal quality cannot be improved not only by the adaptive control but also by the diversity control. Namely, the adaptive antenna apparatus according to the prior art cannot improve the signal quality when the radiation directivity of the antenna has the null. Accordingly, the adaptive antenna apparatus according to the prior art is unavoidably inappropriate.